refactor(server): remove bootstrap admin path

The bootstrap-admin-from-env-vars flow was a holdover from before
self-registration could create the first admin. Now that handleRegister
+ CreateUserFirstAdminRace promotes the first user to register on an
empty users table (verified shipped in v2026.05.08.2 / #376), the
bootstrap path is just a second source of truth that confuses operators
(and leaves an "admin" account in the DB that nobody asked for).

Removes:
- internal/auth/bootstrap.go + its test
- The auth.Bootstrap call from cmd/minstrel/main.go
- AuthConfig + AdminBootstrapConfig structs from config
- MINSTREL_AUTH_ADMIN_USERNAME / _PASSWORD env reads + their test
- The auth: block from config.example.yaml
- Bootstrap-related comments in docker-compose.yml + README.md

The README quickstart now points operators at /register on first start
instead of "watch the logs for a one-time password."

Existing instances keep their bootstrap-admin row in the DB; operators
who want it gone can register a new admin via /register, promote them
in /admin/users, then delete the old bootstrap user (last-admin guard
will require the new admin to be promoted first). No migration needed.

Recovery story for forgotten admin passwords now hinges on Fable #321
(admin password reset CLI) — currently the only path back in if no
other admin exists.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config.example.yaml

@@ -22,16 +22,6 @@ log:
   level: "info"    # debug | info | warn | error
   format: "text"   # text | json
 
-auth:
-  # One-shot admin bootstrap. Applied only when the users table is empty.
-  # Username defaults to "admin" when unset — you can omit this section
-  # entirely for a working bootstrap. Leave password blank to have the server
-  # generate one and log it to stderr on first start.
-  # Env: MINSTREL_AUTH_ADMIN_USERNAME, MINSTREL_AUTH_ADMIN_PASSWORD
-  admin_bootstrap:
-    username: "admin"
-    password: ""
-
 library:
   # Filesystem roots to scan for music. Each path is walked recursively.
   # Env: MINSTREL_LIBRARY_SCAN_PATHS (colon-separated, PATH-style)